The present invention relates to gaseous halogenation reactions and reactors useful in those reactions.
It is well known to react propylene and chlorine in a vapor phase at high temperatures to make a mixture of products which contain predominantly allyl chloride. See, for example, Samples et al., U.S. Pat. No. 3,054,831 (Sep. 18, 1962), which is incorporated herein by reference. In such reactions, propylene and chlorine react at a temperature of 400.degree. C. to 500.degree. C. The molar ratio of propylene to chlorine in the reactor is usually between about 3:1 and about 5:1. The conversion of chlorine is usually about 100 percent, and the conversion of propylene is usually about 20 to 35 percent.
After the propylene is separated from the reaction mixture, the reaction product usually contains about 70 to 80 weight percent allyl chloride with the remainder being predominantly a mixture of many different chlorinated alkanes and alkenes. The reaction also makes small amounts of carbon. The carbon builds up in the reactor over a period of time, until the reactor must be shut down for cleaning.
The mixture of products is temperature dependent. Temperatures below about 400.degree. C. favor the formation of excessive dihalogenated by-products, whereas temperatures more than about 500.degree. C. favor decomposition of allyl chloride to produce excessive carbon and other products. See, for example, Samples et al., supra, at column 1, lines 15-25; and British Patent Specification 761,831 (published Nov. 21, 1956) at column 1, lines 28-40. Therefore, temperature control is important in order to obtain a desirable mixture of products.
However, temperature control is difficult because the reaction is strongly exothermic. The reagent must be injected into the reactor at a temperature far below the desired reaction temperature or the heat of reaction will drive the temperature in the reactor too high. Even so, hot and cool spots may exist in the reactor which generate undesirably high levels of reaction by-products.
Several different methods of improved mixing in the reactor have been suggested to minimize temperature differences.
(1) Vandijk, U.S. Pat. No. 2,763,699 (published Sep. 18, 1956); British Patent Specification 761,831 (published Nov. 21, 1956); and British Patent Specification 765,764 (published Jan. 9, 1957), which are incorporated herein by reference, teach a variety of spherical, egg-shaped, oval and similar reactors which can be used to make allyl chloride. These spherical reactors are still susceptible to fouling from carbon. British Patent Specification 761,831 shows high yields of allyl chloride produced using a series of three spherical reactors. The series of three spherical reactors is inefficient because it is necessary to constantly cool and then reheat the reaction mixture as it passes from one reactor to the other. It is also particularly susceptible to fouling with carbon because carbon produced in the first reactors must pass through the narrow injectors and pipes of subsequent reactors. PA1 (2) Samples et al., U.S. Pat. No. 3,054,831 teaches a complex injection system to encourage turbulence and mixing within the reactor. PA1 (3) Yamamoto et al., Japanese Published Application 48-26732 (published Aug. 15, 1973) teaches a circular tubular reactor with baffles near the injector for reagents to encourage mixing. PA1 (4) Spadlo et al., Polish Patent 136,334 (published Feb. 20, 1987) teaches to premix the reagents at low temperature before they are injected into the reactor. PA1 (1) partially reacting propylene and molecular halogen in a molar ratio of at least about 2.5:1 in a "continuously stirred tank reactor" (CSTR) zone under conditions suitable to provide a reaction temperature of about 400.degree. C. to 525.degree. C. to partially convert propylene and chlorine into allyl chloride; and PA1 (2) feeding an effluent from Step (1) into a plug-flow reactor zone where the reaction is continued at a temperature of about 400.degree. C. to 525.degree. C. until essentially all of the chlorine is consumed. PA1 (1) an approximately spherical, egg-shaped or oval reactor zone; PA1 (2) a tubular reactor zone attached to the spherical, egg-shaped or oval reactor zone; PA1 (3) one or more inlets for injecting gaseous reagents into the spherical, oval or egg-shaped reactor zone; and PA1 (4) one or more outlets from the tubular reactor zone for withdrawing a gaseous product.
All of these reactors are still susceptible to carbon formation. They must be shut down periodically for cleaning. If carbon formation were reduced, the reactor could run longer between shutdowns. What is needed is a reactor and/or process which is highly selective to allyl chloride and which produces very low levels of carbon without the need to continually heat and cool the reaction products.